1. Field of the Invention
This invention relates to micro electro mechanical system (MEMS) devices, and more particularly to hybrid method for fabricating MEMS devices and an optical MEMS device fabricated using the new method.
2. Description of the Related Art
There has been a great deal of recent interest in the development of optical MEMS devices for applications such as projection devices, displays, fiber switches, sensors, beam steering and optical data storage devices. Often these optical MEMS devices are fabricated in array configurations with each of the array elements having a micro-mirror that is individually controllable to reflect light in different directions.
One common process for manufacturing MEMS devices is by bulk micromachining using deep etch processing, which is considered a subtractive fabrication technique because it involves etching away material from a single substrate layer to form the MEMS structure. One of the more common substrate materials is silicon (Si), which provides a good reflective surface for optical MEMS devices. The substrate layer can be relatively thick, on the order of tens of microns, and the sophistication of this process allows for the micromachining of different structures in the substrate such as cantilevers, bridges, trenches, cavities, nozzles and membranes. MEMS devices fabricated using this process are considered to be more robust compared to devices from other processes and they are less subject to the surface tension and forces that act upon MEMS devices during processing. However, because only one layer of material is used, the complexity of the MEMS device is typically limited. Details for this process are discussed in J. K. Bhardwaj, H. J. Ashraf, Advanced Silicon Etching Using High Density Plasmas, Proc. SPIE, vol. 2639, Pg. 224 (1996 SPIE, Bellingham, Wash.), and A. A. Aryon et al. Etching Characteristics and Profile Control in a Time Multiplexed Inductively Coupled Plasma Etcher, Solid State Sensor and Actuator Workshop, pg. 41 (June 1998, Hilton Head, S.C.)
The most common method of fabricating MEMS devices is by surface micromachining techniques. It is considered an additive process because alternate structural layers and sacrificial spacer layers are xe2x80x9cbuilt-upxe2x80x9d to construct the MEMS structure with the necessary mechanical and electrical characteristics. Polycrystalline silicon (polysilicon) is the most commonly used structural material and silicon dioxide (oxide) glass is the most commonly used sacrificial material. In traditional micromachining processes, these layers are formed in polysilicon/oxide pairs on a silicon substrate isolated with a layer of silicon nitride. The layers are patterned using photolithography technology to form intricate structures such as motors, gears, mirrors, and beams. As the layers are built up, cuts are made through the oxide layers and filled with polysilicon to anchor the upper structural layers to the substrate or to the underlying structural layer. After the build-up process, the sacrificial (oxide) layers are removed using various techniques such as hydrofluoric acid release etching, which frees the device to move relative to the substrate. [M. A. Michalicek, J. H. Comtois, and H. K. Schriner, Design and Fabrication of Optical MEMS Using a Four-level, Planarized, Surface Micromachined Polysilicon Process, Proc. SPIE Vol. 3276, pp. 48-55 (1998)].
MEMS devices fabricated using this process can be more complex than bulk micromachined devices, with the complexity of the devices determined by the number of polysilicon/oxide layer pairs. A single pair limits designers to simple sensors. Geared mechanisms require two pairs wherein the polysilicon layer from the first pair is used to form the gears and the polysilicon layer from the second pair used to form the locking hub. Motorized mechanisms require a minimum of three independent layer pairs. More recently, a 5-level surface micromachining technology has been developed, which allows for the fabrication of complex movable components on translatable stages that can engage and interact with other subassemblies. This technology is commonly referred to as the Sandia Ultra-planar Multi-level MEMS Technology V (SUMMiT V). [M. S. Rogers and J. J. Sniegowski, Designing Microelctromechanical Systems-On-AChip in a 5-level Surface Micromachine Technology, 2nd Annual Int. Conf. on Engineering Design and Automation (August 1998), and M. S. Rogers and J. J. Sniegowski, 5-Level Polysilicon Surface Micromachining Technology Application to Complex Mechanical Systems, Proc. 1998 Solid State Sensor and Actuator Workshop, pg. 144 (June 1998, Hilton Head, S.C.)]
One disadvantage of the use of the surface micromachining process for micro-mirror arrays is that the thin polysilicon layers are typically used as the micro-mirrors and they can be extremely sensitive to residual stress. This can cause significant curvature in the micro-mirror, which degrades the optical quality of the individual mirror and of the array. In addition, polysilicon micro-mirrors provide a poor reflective surface and to make them more reflective the mirror surface can be metalized with reflective thin films. However, this can introduce additional film stresses that can cause further micro-mirror curvature. Also, using one of the polysilicon layers for the mirror reduces the number of available structural layers that can be used for actuators, electrodes, springs, etc., reducing the design flexibility and ultimate device complexity.
Finally, the roughness of the polysilicon surface can degrade optical quality. This can be mitigated with surface polishing (as is used in SUMMiT V), or through flip-chip transfer processing. One recently developed optical MEMS device has been fabricated using this flip-chip technique. is [M. A. Michalicek et al., Micromirror Arrays Fabricated by Flip-Chip Assembly, SPIE vol. 3878, (Sept. 20, 1999)]. A silicon mirror is anchored to a silicon substrate by an oxide using commercial foundry processes and bonding pads are deposited on it. A ceramic substrate is then patterned with gold wire and indium bonding pads to mate with the mirror pads. The mirror/substrate structure is then flip-chip mounted on the ceramic substrate and the mirror is released from the substrate by dissolving the oxide with a hydroflouric acid etch. The resulting silicon mirror surface is smooth, replicating the original substrate, but the mirror is very thin. If it is coated to make more reflective, the mirror can be stressed and deformed. Also, variations in bonding stresses can impair global micro-mirror array uniformity.
Another type of optical MEMS array has been developed by first fabricating the array""s drive electronics and then building micro-mirrors on the electronics. [L. J. Hornbeck, Digital Light Processing and MEMS: Timely Convergence for a Bright Future, Plenary Session, SPIE Micromachining and Microfabrication, Vol. 2639, Pg. 2 (October 1995, Austin, Tex.)]. This fabrication process requires low temperatures and is strictly a surface micromachining process This limits the flexibility in changing the process to fabricate different structures for different applications. Also, like the polysilicon micro-mirrors, these micro-mirrors are relatively thin and can be susceptible to deformation under stress. For instance, any coating placed on the mirror to increase its ability to reflect must be very thin or the mirror can deform. Also, by being so thin there is a danger that the mirrors can misregister, degrading the array""s performance.
The present invention provides a new method for fabricating MEMS devices and a new optical MEMS device fabricated using the new method. The new method is a hybrid that combines bulk deep-etch and surface micromachining fabrication techniques to include the advantages of both.
The new hybrid method includes the step of providing a layer of bulk micromachining material and then building up layers of structural and sacrificial material on the bulk micromachining material using surface micromachining techniques. A substrate with drive electronics is then mounted to the layers of structural and sacrificial material. The bulk material is then micromachined and the sacrificial material within the various surface layers is dissolved.
The new method is particularly applicable to fabricating optical MEMS devices, beginning with the step of mounting a handle layer to a mirror layer, to hold and protect the mirror layer. Layers of structural and sacrificial materials are then built on the mirror layer using surface micromachining techniques, with the structural material forming electrode and spring structures. Drive electronics are then mounted on the layers of structural material so that a bias can be applied to the electrode and spring structures The handle layer is removed from the mirror layer to reveal the mirror""s reflective surface, and the sacrificial material is dissolved from the surface micromachining layers, freeing the electrode and spring structures to operate.
For optical or other MEMS device arrays, a linking framework can be included to link the individual MEMS elements to one another. In the above method, the linking framework is preferably built-up on the layers of structural and sacrificial material prior to mounting the drive electronics. The linking framework is also built up using the same surface micromachining techniques. The drive electronics are then mounted to the linking framework to provide mechanical and electrical interconnection with the MEMS structure.
The invention also provides a new optical MEMS device fabricated using a hybrid fabrication process including a mirror fabricated using bulk micromachining techniques and a substrate with drive electronics to activate the mirror. The device also includes a spring structure and an electrode fabricated using surface micromachining techniques. The spring structure is connected between the substrate and mirror, holding the mirror above the substrate. The electrode is mounted on the substrate between said mirror and said substrate. A bias applied across the mirror and electrode activates the device, causing the mirror to be drawn toward the electrode against the action of the spring structure.
The new hybrid method and new optical MEMS device provide many advantages over conventional methods and devices. For optical MEMS devices the micro-mirror is formed from the subtractive bulk deep etch process wherein the handle layer is removed from the Si layer. The resulting micro-mirror is thicker and more rugged than those formed from surface micromachining processes and provides a good reflective surface. The device also includes MEMS structures that are built-up using surface micromachining techniques. This allows for the fabrication of more complex MEMS structures compared to purely bulk micromachined devices. By including a linking framework for MEMS arrays, global uniformity can be enhanced.
These and other further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which: